1. Field of the Invention
The present invention relates to a control system for an internal combustion engine installed on a vehicle, and more particularly to a control system for an internal combustion engine, for executing idle stop control for restarting the engine after temporarily stopping the engine during stoppage of the vehicle.
2. Description of the Related Art
Conventionally, the present applicant has already proposed a control system disclosed in Japanese Patent Publication No. 4475437 as a control system for an internal combustion engine. This engine is installed on a hybrid vehicle as a motive power source. The vehicle is equipped with a motor generator as a motive power source, an air conditioner, and so forth. Further, the control system executes operation control of the air conditioner, idle stop control of the engine, and so forth, and is equipped with various sensors, such as a humidity sensor, and various switches, such as an air conditioner switch.
In this control system, the idle stop control is executed such that while the air conditioner is performing a cooling operation during stoppage of the vehicle, the engine is stopped when predetermined stop conditions are satisfied, and the engine is restarted when a time period elapsed after the stoppage of the engine has reached an engine stoppage time Ts. This engine stoppage time Ts is calculated by a method shown in FIGS. 2 and 3 in Japanese Patent. Publication No. 4475437. More specifically, a temperature rise coefficient Kt is calculated according to the amount of air blown out from the air conditioner and a state of settings of the air conditioner switch for inside air circulation or outside air introduction, and an allowable upper limit temperature Eu of an evaporator is calculated by searching a map according to a target blowout temperature.
Then, a compartment temperature rise-permitting time Tu is calculated as the reciprocal of a value obtained by multiplying a difference between the allowable upper limit temperature Eu and a temperature Es of the evaporator during stoppage of the engine by the temperature rise coefficient Kt. Then, a fogging occurrence estimated time Tf is calculated based on a humidity or the like within a vehicle compartment during stoppage of the engine. When Tf<Tu holds, the engine stoppage time Ts is set to the compartment temperature rise-permitting time Tu, whereas when Tf≧Tu holds, the engine stoppage time Ts is set to the fogging occurrence estimated time Tf.
In the case of the control system disclosed in Japanese Patent Publication No. 4475437, a control method is employed in which the idle stop control is executed during the cooling operation of the air conditioner, and hence there is a problem that the control method cannot be applied to idle stop control during a heating operation of the air conditioner. For example, although in the control system disclosed in Japanese Patent Publication No. 4475437, the engine stoppage time Ts is calculated using e.g. the temperature of the evaporator, the evaporator is used during the cooling and dehumidifying operations of the air conditioner, but during the heating operation of the air conditioner, the evaporator is held at rest. without being used. Therefore, during the heating operation of the air conditioner, when the engine stoppage time Ts is calculated using the temperature of the evaporator as a parameter as in the control system disclosed in Japanese Patent Publication No. 4475437, there is a fear that the calculated engine stoppage time Ts becomes improper. For example, when a too long time period is obtained as a result of the calculation, the temperature of the vehicle compartment is lowered to an unpleasant temperature before the restart of the engine after stoppage thereof. On the other hand, when a too short time period is obtained as a result of the calculation, the time period before the restart of the engine after stoppage thereof is made shorter than required, which degrades fuel economy.